General direct current motors have a rotor which is an armature having a number of coils, and a stator which is placed outside this rotor in the radial direction. The stator has a number of field poles. A commutator having a number of segments is attached and integrated with the rotary shaft of the rotor in such a manner as to be rotatable. The stator has a brush which makes contact with and slides against the above described commutator so that electricity is supplied. When the motor is driven, electricity is supplied to the number of coils of the rotor in sequence via the brush and the commutator.
In addition, there are motors in which an armature having a number of coils functions as a stator instead of a rotor, and this armature stator is placed outside the rotor in the radial direction. This type of motor generally has a control circuit for switching the supply of a current to the number of coils in sequence. In contrast, Patent Document 1 discloses a motor in which the supply of a current is switched without using a control circuit.
The direct current motor disclosed in the above described document has an armature stator having nine coils and a rotor having a number of field poles which is placed inside the armature stator in the radial direction. A rotational disc is secured to the rotary shaft of the rotor. The rotational disc has a commutator which is placed in the vicinity of the center and three slip rings in concentric circles, large, medium and small, surrounding the commutator. The commutator is made of a number of commutator pieces aligned in the circumferential direction, and each commutator piece is connected to one of the three slip rings. One of the three slip rings functions as an anode slip ring, and another functions as a cathode slip ring.
The above described armature stator has a pair of first brushes which make contact with and slide against the above described commutator, and a total of three second brushes which respectively make contact with and slide against the above described slip rings. One of the pair of first brushes functions as an anode brush, and the other functions as a cathode brush. One of the three second brushes functions as an anode brush, and another functions as a cathode brush. Out of the nine coils, every three continuing in the circumferential direction are connected in series. That is to say, the nine coils are divided into three groups.
In the direct current motor in the above described document, a current flows through an electrical path starting from an external power supply, passing through a first anode brush of the armature stator, an anode slip ring of the rotor, a second anode brush of the armature stator, a coil, a second cathode brush, a cathode slip ring of the rotor and a first anode brush of the armature stator, and reaching the external power supply.
The rotor rotates due to the interactions between the rotational magnetic fields generated by the coils to which a current is supplied, and the magnetic field generated by the field poles of the rotor. The rotational disc rotates together with the rotor, and at the same time, the commutator piece making contact with and sliding against a first brush secured to the armature stator is switched in sequence, so that the three serial, connected coils through which a current flows are also switched in turn.
The motor in the above described document has such a configuration that currents having the same phase are supplied to three coils which continue in the circumferential direction in a group from among the nine coils, and when the motor is driven, currents having different phases are simultaneously supplied to two groups, so that rotational magnetic fields are generated. In a direct current motor having such coils, the field poles of the rotor are generally set so that the angular interval between respective N poles and S poles is 180°. That is to say, it can be said that the direct current motor of the above described document is driven in accordance with the same principle as motors having two field poles and three coils.
In the direct current motor in the above described document, two slip rings which are used to supply a current to the armature stator are selected from three slip rings, and therefore, no current is supplied to the three coils which are connected to the unselected slip ring. That is to say, when the direct current motor is driven, a current flows through six out of the nine coils, and at all times, there are three coils to which no current is supplied, and therefore, the efficiency of use of the coils is low. When the efficiency of use of the coils is low, the output of the direct current motor also becomes low.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-230657